How it works:
Dial the codes below into the phone app on your device - the (SEND) button will be the button you normally use to start a call

Forward ALL incoming calls to voicemail when roaming overseas
Activate: * * 21 * +6421700700 # (SEND)
Deactivate: # # 21 # (SEND)

Forward ALL incoming calls immediately
Activate: * * 21 * full phone number # (SEND)
Deactivate: # # 21 # (SEND)

Forward unanswered calls - default is Voicemail +6421700700
Activate: * * 61 * full phone number # (SEND)
Deactivate: # # 61 # (SEND)

Forward calls when out of coverage or phone switched off - default is Voicemail +6421700700
Activate: * * 62 * full phone number # (SEND)
Deactivate: # # 62 # (SEND)

Forward calls when busy - default is Voicemail +6421700700
Activate: * * 67 * full phone number # (SEND)
Deactivate: # # 67 # (SEND)

Pro Tip - If you regularly divert your mobile to another number, save these codes as contacts to save typing them out each time.